Integrating the role of relative density on assessing internal stability of granular filters using existing geometrical methods

The filtrates under the influence of strong seepage forces accompany finer fractions from broadly graded granular filters, occasionally rendering them ineffective. This phenomenon is termed as internal instability, and its prior assessment in practice is emphasized through some of the well-known geometrical criteria which examine only the shape of the particle size distribution and ignore the level of compaction of soils. In this study, gradient-controlled hydraulic tests conducted over a range of compacted soils were used to analyze some of the geometrical methods, which showed partial success in assessing the stability of test specimens. Further analysis facilitated a useful revision of a well-accepted criterion, whereby the original stability boundaries based on the uniformity coefficient could be set on the basis of relative density to quantify the potential of internal stability with enhanced accuracy. Not surprisingly, all the tests performed in this study along with additional data from published literature validated the revised method. Moreover, the proposed method is endorsed through a real-life design example, thus making it more expedient for practicing engineers.